This invention relates to fuel-cell stacks having external manifolds and, in particular, to a retention system for maintaining the external manifolds in sealing relationship to the fuel-cell stack.
A fuel-cell is a device which transforms chemical energy in the form of fuel (e.g., natural gas, bio-gas, methanol, diesel fuel, etc.) directly into electrical energy by way of an electrochemical reaction. Like a battery, a fuel-cell contains two electrodes, an anode and a cathode. Unlike a battery the fuel-cell will produce electrical power as long as fuel and oxidant are delivered to the anode and cathode, respectively. The major advantage of fuel-cells over more traditional power generation technologies (e.g., IC engine generators, gas or steam turbines, etc.) is that the fuel-cell converts chemical to electrical energy without combusting the fuel. The efficiency of the fuel-cell is, therefore, not thermodynamically limited, as are heat engines, by the Carnot cycle. This allows fuel-cell based systems to operate at a far higher efficiency than traditional power plants thereby reducing fuel usage and byproduct emissions. Additionally, due to the controlled nature and relatively low temperature of the chemical reactions in a fuel-cell, the system produces nearly zero pollutant emissions of hydrocarbons, carbon monoxide, nitrogen oxides and sulfur oxides.
Fuel-cells are typically arranged in stacked relationship. A fuel-cell stack includes many individual cells and may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel-cell plates themselves. In an externally manifolded stack, the fuel-cell plates are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel-cell stack. The manifolds thus provide sealed passages for delivering fuel and oxidant gasses to the fuel-cells, thereby preventing those gasses from leaking either to the environment or to the other manifolds. The manifolds must perform this function under the conditions required for operation of the fuel-cell and for the duration of its life.
An important aspect of the performance of a fuel-cell stack manifold is the gas seal established between the manifold edge and the stack face. As the stack face is typically electrically conductive and has an electrical potential gradient along its length and the manifold is typically constructed from metal, a dielectric insulator is needed to isolate the manifolds from the fuel-cell stack. As the dielectric insulator is typically constructed from ceramic which tends to be brittle, care must be taken in how the manifolds are compressed against the stack face so as not to damage the dielectrics.
Another requirement of fuel-cell stack manifolds relates to the fact that typically a fuel-cell stack will shrink over its life as the cell components creep and densify at high temperature. For a tall fuel-cell stack the total height may decrease by 2-3 inches. This means that continuous, metal manifolds cannot be fixed to both the top and bottom of the stack but must, instead, be allowed to slide relative to the stack face. Therefore, the retention system employed to compress the manifolds against the fuel-cell stack must maintain adequate normal force to prevent gas leakage while allowing the manifolds to slide along the stack face. In addition, the forces cannot be so great that the dielectric insulators are caused to break.
Due to its inherently non-uniform temperature distribution, a tall fuel-cell stack also tends to bow. Horizontal deflection of the top of the stack at high temperature can be as much as 1-2 inches relative to the base of the stack. This places a further burden on the manifolds and retention system which are required to flex with the bowing stack in order to maintain tight gas seals.
Fuel-cells operate at temperatures above ambient (Polymer Electrolyte Fuel-cells, xe2x80x9cPEFCxe2x80x9d: xcx9c80xc2x0 C.; Phosphoric Acid Fuel-cells, xe2x80x9cPAFCxe2x80x9d: xcx9c200xc2x0; Molten Carbonate Fuel-cells, xe2x80x9cMCFCxe2x80x9d: xcx9c650xc2x0 C.; Solid Oxide Fuel-cells, xe2x80x9cSOFCxe2x80x9d: xcx9c1000xc2x0 C.). Therefore, the selection of materials and the mechanical design must allow the components to last for the life of the fuel-cell stack (typically years). Component stress and corrosion must be considered relative to the environment in which these components must perform. In the case of MCFC and SOFC the temperatures are high enough and the lifetime long enough that long term creep of metallic components must be considered in their design.
The current state of the art fuel-cell manifold retention system used by the assignee of the subject application for carbonate fuel-cell stacks includes rigid mechanical members and springs which attempt to apply exclusively normal load to the rails of the manifolds. The components are constructed from high-temperature, corrosion-resistant materials such as nickel-based alloys and stainless steels. Leaf springs are used to transfer the tension developed in the rigid members to the manifold rails in multiple locations. This is done to provide as uniform a pressure under the manifold rails as possible. As the leaf springs see a relatively high stress level, they are intricately designed with tight dimensional tolerances so as to minimize this stress and extend their life.
The current design uses a large quantity of different parts to satisfy the requirements for a uniformly distributed normal load application to the manifold as well as to allow both stack shrinkage and stack bowing. The selected materials, intricacy of the geometry and large number of parts used in this design make it expensive, heavy and difficult to install. Also, the current system is designed to function completely independently from the manifolds and, as such, results in certain redundancies of material which add to the cost, weight and complexity of the fuel-cell stack.
Other fuel-cell stack manifold retention systems are described in various issued patents. A number of these patents are listed below:
It is, therefore, an object of the present invention to provide a fuel-cell stack manifold retention system which does not suffer from the above disadvantages.
It is a further object of the present invention to provide a fuel-cell stack manifold retention system which is less costly, less complex and easier to assemble.
It also an object of the present invention to provide a fuel-cell stack manifold retention system which better allows for stack bowing and stack shrinkage.
It is yet another object of the present invention to provide a fuel-cell stack retention system which is lighter in weight and results in more effective gas sealing.
In accordance with the principles of the present invention, the above and other objectives are realized in a fuel-cell stack manifold retention system which includes a number of strap members for retaining the stack manifolds against the corresponding stack faces. Each strap member extends over an extent of a manifold and an attachment assembly attaches the strap members to the fuel-cell stack. The strap members are further adapted to expand, contract and slide. In this way the strap members are able to hold the manifolds against the faces of the fuel-cell stack so as to accommodate changes in the fuel-cell stack geometry caused by temperature changes and material creep.
In the embodiment of the invention to be disclosed hereinbelow, each strap member is connected to one manifold, extends around the adjacent manifold and the attachment assembly connects the strap members to the opposite side of the stack. Also, in the disclosed embodiment, the strap members are spring loaded to provide the expansion and contraction. Additionally, the attachment assembly includes truss members for supporting the length of the strap members extending over the manifolds and yoke assemblies and hook members for holding the ends of the strap members. Certain of these components are fixedly secured to the manifolds, as by welding, or they can be designed as part of the manifolds themselves.